reserveOuterNoun

nextIndex

diverge

Enabled: Return a new non-mutable OuterScope that does not share outers data
structures with the receiver.

ImportLoader is
potentially used from multiple vats, and so must be thread-safe. An
ImportLoader holds an OuterScope only for the purpose of diverging new
OuterScopes per emaker eval. Therefore, the state that can be shared
between diverged scopes needs to be thread-safe. That state is
myScopeMap, and ScopeMaps are indeed thread-safe.